In the conventional method of making electrostatographic toner powders, a binder polymer and other ingredients, such as a pigment and a charge control agent, are melt blended on a heated roll or in an extruder. The resulting solidified blend is then ground or pulverized to form a powder. Inherent in this conventional process are certain drawbacks. For example, the binder polymer must be brittle to facilitate grinding. Improved grinding can be achieved at lower molecular weight of the polymeric binder. However, low molecular weight binders have several disadvantages; they tend to form toner/developer flakes; they promote scumming of the carrier particles that are admixed with the toner powder for electrostatographic developer compositions; their low melt elasticity increases the off-set of toner to the hot fuser rollers of the electrostatographic copying apparatus, and the glass transition temperature (Tg) of the binder polymer is difficult to control. In addition, grinding of the polymer results in a wide particle size distribution. Consequently, the yield of useful toner is lower and manufacturing cost is, therefore, higher. Also the toner fines accumulate in the developer station of the copying apparatus and adversely affect the developer life.
The preparation of toner polymer powders from a preformed polymer by the process known as "evaporative limited coalescence" offers many advantages over the conventional grinding method of producing toner particles. In this process, polymer particles of small size and narrow size distribution are obtained by forming a solution of a polymer in a solvent that is immiscible with water, dispersing the solution so formed in an aqueous medium containing a solid colloidal stabilizer such as colloidal silica, and removing the solvent by evaporation. The resultant polymer particles are then isolated, washed and dried.
Toner particles can be prepared by the evaporative limited coalescence technique from any type of binder polymer that is soluble in a solvent that is immiscible with water. The size and size distribution of the resulting polymer particles can be controlled by the relative quantities of the polymer employed, the solvent, the quantity and size of the water-insoluble particulate suspension stabilizer and the size to which the solvent-polymer droplets are reduced by the agitation employed in dispersing the organic solution in the aqueous medium.
Limited coalescence technology provides a means for overcoming many of the drawbacks of the prior art melt blending and grinding procedures for toner manufacture. By using the limited coalescence method it is possible to obtain narrow particle size distribution with no fines, to use tough binder polymers which would be difficult to pulverize and to use toner ingredients which would be degraded in a melt blending process. Representative patents disclosing toner manufacture by limited coalescence and advantages thereof include U.S. Pat. Nos. 4,833,060; 4,835,084; 4,965,131; and 5,133,992, each of which is incorporated herein by reference.
A characteristic of the toner particles prepared by the prior art method of limited coalescence is that the particles are generally spherical, especially when the particle size is smaller than 10 microns. Spherical particles can provide some advantages in the quality of the image formed. However, the transfer efficiency, i.e., in transferring a developed toner image from the photoconductor to a receiving sheet by electrostatic attraction, is superior for particles of irregular shape. It is believed that irregular particles help in electrostatic transfer of toner by interlocking with neighboring particles having irregular morphology. Accordingly, workers in the art have sought to modify the shape of the normally spherical toners made by evaporative limited coalescence in order to improve the transfer properties of the toner. U.S. Pat. No. 5,283,151 describes one method by which irregularly shaped particles can be obtained by limited coalescence. In the method of the patent, carnauba wax is introduced into the organic phase of the limited coalescence process in a limited amount. The use of the limited amount of this specific wax, i.e., carnauba wax, which is highly surface active, results in the formation of non-spherical toner particles once the solvent is removed.
Although it is possible to produce irregular polymeric particles by the use of carnauba wax as a toner component, the wax cannot be removed from the polymer particles by the method used for removing silica from the polymer particles in conventional limited coalescence methods. i.e., by washing the particles with an aqueous alkaline solution. The retention of the wax on the polymer particles has several disadvantages in the use of the polymeric powder as an electrostatographic toner. For example, it can adversely affect the tribocharging properties, particle cohesiveness, powder flow behavior, sensitivity to environment, scumming of carrier particles or photoconductor, Theological performance, and/or toner keeping behavior.
There is a need, therefore, for a process for making non-spherical, irregular toner particles that offers the benefits of the limited coalescence method but without the undesirable effects of additives such as carnauba wax in the toner formulation.
The present invention provides such a process as well as novel polymeric powders that are useful as electrostatographic toners.